1. Field of the Invention
The present invention relates to a piezoelectric material suitable for a piezoelectric element and an ultrasonic probe which includes the piezoelectric element consisting of the piezoelectric material and is useful in, e.g., a medical diagnosing apparatus.
2. Description of the Related Art
An ultroaonic probe has an ultrasonic transmitting/receiving element having a piezoelectric element. The ultrasonic probe is used for imaging the internal state of a target by radiating an ultrasonic wave toward the target and receiving an echo reflected by an interface having a different acoustic impedance of the target. An ultrasonic imaging apparatus incorporating such an ultrasonic probe is applied to, e.g., a medical diagnosing apparatus for inspecting the interior of a human body and an inspecting apparatus for inspecting the interior of a metal welding portion.
As an example of the medical diagnosing apparatus, in addition to the tomographic image (B mode image) display of the human body, there has been recently developed an apparatus employing the "Color Flow Mapping (CFM) method" capable of performing two-dimensional color display of the speed of the blood flow of, e.g., the heart, liver, and carotid artery, by utilizing a Doppler shift in ultrasonic wave caused by the blood flow. The diagnosing performance has been remarkably improved by this medical diagnosing apparatus. The medical diagnosing apparatus employing the CFM method is used for diagnosis of all the internal organs, e.g., the uterus, liver, and spleen, of the human body. Further studies are in progress aiming at an apparatus capable of diagnosing coronary thrombus.
In the case of the former B mode image, a high-resolution image must be obtained at a high sensitivity so that even a small change to a morbid state and a body cavity at a deep location caused by a change in body can be clearly seen. In the latter Doppler mode capable of obtaining a CFM image, since the echo reflected by a small blood cell having a diameter of about several fm is used, the obtained signal level is lower than that obtained in the B mode image, and thus a higher sensitivity is required.
Conventionally, ultrasonic transmitting/receiving elements having the structures as follows are used in terms of their performance:
(1) Ultrasonic attenuation caused by irradiating a living body with an ultrasonic wave by an ultrasonic probe is about 0.5 to 1 dB/MHz.cm except in bones. Thus, in order to obtain a high-sensitivity signal from the human body, it is preferable to decrease the frequency of the ultrasonic wave radiated by the ultrasonic transmitting/receiving element. When, however, the frequency is excessively decreased, the wavelength of the frequency is increased to sometimes degrade the resolution. Therefore, an ultrasonic wave having a frequency of 2 to 10 MHz is usually radiated. PA1 (2) The piezolelectric element of the ultrasonic transmitting/receiving element must be constituted by a material having a large electromechanical coupling coefficient and a large dielectric constant so that loss caused by cables and the stray capacitance of the apparatus is small and that the piezolelectric element be easily matched with a transmitting/receiving circuit. For this reason, the piezoelectric element is mainly constituted by a lead zirconate titanate (PZT)-based ceramic. PA1 (3) An array-type ultrasonic probe constituted by arranging several tens to about 200 ultrasonic transmitting/receiving elements each having a rectangular piezoelectric element has a high resolution. PA1 a piezoelectric element having an ultrasonic transmitting/receiving surface; and PA1 a pair of electrodes formed on the ultrasonic transmitting/receiving surface of the piezoelectric element and a surface opposite to the transmitting/receiving surface, respectively, and PA1 wherein the piezoelectric element consists of a piezoelectric material containing a composition which is represented by xPb(Sc.sub.1/2 Nb.sub.1/2)O.sub.3 -ypbTiO.sub.3 -zPbZrO.sub.3 -wPb(Me.sub.1/3 Nb.sub.2/3)O.sub.3, where Me is at least one metal selected from the group consisting of Mg, Zn and Ni, and x+y+z+w=1.00, x, y, z, and w being values falling within a region which is defined by connecting points a, b, c and d and which excludes a line ab, the points a, b, c and d existing on the faces of a regular trigonal pyramid having apices P.sub.1, P.sub.2, P.sub.3 and P.sub.4 which correspond to Pb(Sc.sub.1/2 Nb.sub.1/2)O.sub.3, PbTiO.sub.3, PbZrO.sub.3 and Pb(Me.sub.1/3 Nb.sub.2/3)O.sub.3, respectively, and the points a, b, c and d having the following coordinate values when the apices P.sub.1, P.sub.2, P.sub.3 and P.sub.4 have coordinate values of (X.sub.1, Y.sub.1, Z.sub.1, W.sub.1 =1, 0, 0, 0 ), (X.sub.2, Y.sub.2, Z.sub.2, W.sub.2 =0, 1, 0, 0), (X.sub.3, Y.sub.3, Z.sub.3, W.sub.3 =0, 0, 1, 0), and X.sub.4, Y.sub.4, Z.sub.4, W.sub.4 =0, 0, 0, 1):
The above conventional ultrasonic probes, however, have the following problems.
That is, in the array-type ultrasonic probe, the number of piezoelectric elements tends to increase as the resolution of the probe is increased. To bring the ultrasonic probe of this type into contact with a human body, the width of the piezoelectric element must be decreased since the diameter of an ultrasonic radiating surface cannot be increased. A dicer used in cutting semiconductor silicon wafers or the like is used to form rectangular piezoelectric, elements each having a width of 100 .mu.m or smaller from a block of a PZT-based ceramic. Since, however, the piezoelectric element readily cracks during cutting using the dicer, a piezoelectric material with a higher fracture toughness is required.
In addition, if the number of piezoelectric elements is increased in the above ultrasonic probe, the impedance per piezoelectric element rises to make it difficult to obtain impedance matching with the driving circuit. This problem of poor matching can be avoided by use of PZT with a large dielectric constant. However, the electromechanical coupling coefficient of the PZT-based ceramic described above decreases if its permittivity exceeds 3,000. This introduces another problem of a decrease in sensitivity.
As described above, in the manufacturing process of the ultrasonic probe using the PZT as the piezoelectric element, cracks readily occur if the width of the piezoelectric element is decreased to about 100 .mu.m or smaller. In addition, an electromechanical coupling coefficient k.sub.33 ' in the direction of thickness tends to decrease in the above-mentioned rectangular piezoelectric element with a small width.
V. J. TENNERY et al., on the other hand, have reported the use of a Pb(Sc.sub.1/2 Nb.sub.1/2)O.sub.3 -PbTiO.sub.3 -based ceramic composition as a piezoelectric material in place of the PZT in J. AM. CERAM. SOC., VOL. 51, NO. 12, pp. 671-673 (1968). Since, however, the sintering temperature of this ceramic material is extremely high, 1,320.degree. to 1,385.degree. C., a large amount of lead oxide evaporates during sintering, resulting in a low sintering density which is 93% or less of a theoretical density. For this reason, this ceramic composition is not sufficiently satisfactory in respect of fracture toughness. Consequently, cracks occur if a piezoelectric element with a thickness of 100 .mu.m or smaller is cut out from a block of this ceramic composition in order to apply the ceramic composition to an ultrasonic probe. In addition, an electromechanical coupling coefficient k.sub.p of the above piezoelectric element is small, a maximum of at most 46%, which is an unattractive value compared to that (k.sub.p : 60% or higher) of the PZT. It is therefore impossible to use this piezoelectric material as the piezoelectric element of an ultrasonic probe.